As a technology of manufacturing membranes and a membrane application technology have been recently dramatically developed, a membrane technology is widely used in liquid processing fields such as removal of contaminants from liquid, or separation, enrichment and recovery of useful materials from liquid.
Existing membrane technologies are being replaced by membrane technologies because of constant performance and stability according to membrane pore sizes, and due to convenience and concise system according to automation.
There are porous membranes and calendered nonwovens (that is nonwoven fabrics) as membranes used in existing liquid filters.
Porous membranes are produced in a manner that membranes are formed by using polymer materials, for example, PTFE-based nylon, polysulfone, etc., and then pores are formed in the membranes by using chemical and physical methods. Here, since a pore structure is a closed pore structure of the two-dimensional geometry, filter efficiency is low.
In addition, since a pore structure is a closed pore structure of the two-dimensional geometry in the case of using a hydrophobic polymer such as PTFE (polytetrafluoroethylene) in a conventional filter, liquid does not pass through the conventional filter easily. Accordingly, the conventional filter needs to be pressurized. As a result, problems such as high energy costs, frequent filter replacement, and a low flow rate of water are pointed out.
Moreover, since a medium is about 100 μm thick, such a porous membrane is thick and weighs much depending on a material. Thus, there is a problem that it is difficult of bending a porous membrane medium and putting a lot of threads per inch in a filter.
Meanwhile, calendered nonwovens form fibers from polymeric materials, for example, polypropylene, through a melt-blown spinning method, but the size of the fibers is in micro units. Accordingly, unless the fibers have micropores, they are not uniformly distributed and pores are uneven. Also, since contaminants exit concentratively through large pores, filter efficiency is low.
In addition, calendered nonwovens has an average pore size of about 5 μm to about 20 μm, and an excessive calendaring should be performed in order to reduce the average pore size of the filter into about 3 μm or less. However, excessive calendering clogs pores and thus porosity becomes small. Accordingly, if calendered nonwovens are used in liquid filters, a filter pressure becomes high and pores are quickly clogged, to thus cause a negative effect upon a filter life.
Thus, even if a liquid processing module is manufactured by using the existing membrane technology, a fluid flow is lowered due to a membrane clogging phenomenon and a driving pressure rises.
The membrane clogging phenomenon is severe especially in the high concentration fluid, and it was impossible to apply the membrane technology for the high concentration and high turbidity fluid. Also, pores become open to thus cause durability to be lowered.
Therefore, long life and high efficient membranes having consistent filtering performance and reliability according to the pore size with a thin layer of a fine pore structure should be developed urgently so as to be used for liquid processing.
On the other hand, the Korean Laid-Open Patent Publication No. 2008-60263 proposed a filter medium including one or more nanofiber layers of polymer nanofibers having an average fiber diameter of approximately 1 μm or less, in which a mean flow pore size is in the range of about 0.5 μm to about 5.0 μm and solidity is in the range of about 15% to about 90% by volume, and a water flow rate through the medium is greater than about 0.055 L/min/cm2 at a differential pressure of about 10 psi (about 69 kPa).
The method of manufacturing a filter medium proposed in the Korean Laid-Open Patent Publication No. 2008-60263, includes a spinning beam unit having one or more spinning beams each having a spinning nozzle, a blowing gas injection nozzle, and a collector, and is characterized in that a polymer solution is compressed and discharged from the spinning nozzle by using a fine fiber spinning apparatus in which a high voltage electric field is maintained between the spinning beam and the collector, and simultaneously the compressed and discharged polymer solution is blown together with a blowing gas discharged from the blowing gas injection nozzle, to thus form a fiber web of nanofibers and collect the formed fiber web in a single passage moving collection device below a single spinning beam.
Also, a formic acid solution containing nylon of about 24 wt % is used as the polymer solution in the Korean Laid-Open Patent Publication No. 2008-60263, to thus spin nanofibers by using an electro-blown spinning method or an electroblowing method, and to thereby form a web.
However, the method of forming a fiber web of nanofibers in the Korean Laid-Open Patent Publication No. 2008-60263 cannot be called a manufacturing technology of using a multi-hole spin pack. In addition, in the case of producing a nanofiber web by an air-electrospinning (AES) method in an air spraying air-electrospinning apparatus using a multi-hole spin pack having a large number of spinning nozzles arrayed in a large number of rows and columns in order to increase productivity, in which air spinning takes place in each nozzle, a spinning solution containing a polymer of about 24 wt % increases viscosity. As a result, since solidification takes place at the surface of the solution, it is difficult to perform spinning for a long time. Also, since fibers increase in diameter, a fiber web of micrometers or less cannot be formed.
Furthermore, in the case that the ultrafine fiber web obtained by electrospinning does not go through a pretreatment process of appropriately adjusting the amount of the solvent and moisture remaining on the surface of the web before performing calendering, pores are increased but the strength of the web is weakened. Otherwise, in the case that evaporation of the solvent is not performed too slowly, a phenomenon of melting the web may occur.